In the papermaking industry, the drying batteries represent, within the installations, the highest and most costly energy consumption (thermal energy) of the manufacturing process.
The elimination of water in the papermaking process has been accomplished by three different ways, depending on the physicomechanical state of the sheet.
In the first step, the sheet has some very weak mechanical features or properties, thus the removal of water is by filtration over a continuous mesh. In other words, the water abandons the sheet by gravity or natural dripping, escaping between the tangle that the cellulose fibers form.
In the second state of the process, the sheet is more resistant and is subjected to different successive pressings whose pressure values are progressive. The limit of water removed in this state has been marked for years by the resistance itself of the sheet with regard to high pressures. Thus the pressing action managed to achieve moisture limits in the sheet from 6.0 to 56%.
Recent technological advances in pressing have made it possible to reduce this index and approach values of 50%.
Any present system fails in obtaining a greater dryness of the sheet.
In the conventional techniques used for the drying of paper, the elimination of water in the sheet is accomplished by evaporation, by means of a battery of drying drums heated inside by steam at pressures which vary between 0-8M P/M.sup.2.
The sheet of paper is led through the dryers, remaining in contact with their surface, permitting the evaporation of the water contained in the paper upon producing a heat transfer from the surface of the dryer to the sheet of paper.
Independent of other existing drying systems (drying tunnels) the sequence of water contained in the sheet, depending on the type of paper and manufacturing method, is approximately the following:
______________________________________ WATER FOUND PROCESS IN THE SHEET ______________________________________ Formation: 99% Entry into the pressing system 80% Emergency from the pressing system 55-50% and entry into the drying instal- lation ______________________________________